1. Field of the Invention
The present invention relates to a base for a magnetic recording medium, and a HDD (Hard Disk Drive) having a plurality of magnetic recording media using such a base for the magnetic recording medium.
2. Description of the Related Art
There is demand to considerably improve recording density (or storage capacity) of magnetic recording media that are used in HDDs. Particularly, introduction of a MR (Magneto-Resistive) head and the PRML (Partial Response Maximum Likelihood) technique has further improved in-plane recording density of magnetic recording media.
On the other hand, due to recent developments in the spheres of the Internet and big data utilization, data storage capacities of data centers continue to increase. Due to space limitations at data centers, there are demands to increase storage capacity per unit volume. In other words, in order to increase the storage capacity of one standardized HDD, there are proposals to increase a number of magnetic recording media accommodated within a casing of a standardized HDD.
An Al alloy substrate or a glass substrate is primarily used as a base for magnetic recording media. Compared to glass substrates, Al alloy substrates have a higher toughness and are easier to manufacture, and thus, Al alloy substrates are used for magnetic recording media having a relatively large diameter. A thickness of an Al alloy substrate used for a magnetic recording medium of a 3.5-inch standardized HDD is 1.27 mm, for example.
However, when the thickness of the base for the magnetic recording medium is reduced in order to increase the number of magnetic recording media that can be accommodated within a casing of the HDD, fluttering more easily occurs in the case in which an Al alloy substrate is used when compared to the case in which a glass substrate is used. Fluttering of a magnetic recording medium occurs when the magnetic recording medium is rotated at a high speed. When fluttering occurs, it becomes difficult to stably read information from the magnetic recording medium within the HDD.
In the case in which a glass substrate is used, for example, the base for the magnetic recording medium is made of a material having a high Young's modulus in order to reduce fluttering, as proposed in Japanese Laid-Open Patent Publication No. 2015-26414, for example.
In addition, there is a proposal to fill the inside of the casing of the 3.5-inch standardized HDD with helium gas in order to reduce fluttering of magnetic recording media. According to this proposal, 6 or more magnetic recording media can be accommodated within the casing, by reducing the thickness of the aluminum alloy substrate.
Generally, the Al alloy substrate is manufactured as follows. First, an Al alloy plate having a thickness on the order of 2 mm or less is formed into a donut-shaped substrate having a desired size by punching. Next, inner and outer peripheral edges of the donut-shaped substrate are chamfered, and data recording surfaces of the donut-shaped substrate are subjected to machining by rotation. Surface roughness and undulation of the data recording surfaces are reduced by grinding using a grinding stone. Thereafter, from a viewpoint of providing surface hardness and reducing surface defects, the substrate surfaces are NiP-plated. Next, both substrate surfaces (or data recording surfaces), having the plated NiP film, are subjected to polishing, to form the base for the magnetic recording medium.
The bases for the magnetic recording media are mass-produced, and high cost performances are required of the bases for the magnetic recording media. For this reason, the Al alloys used for the bases for the magnetic recording media are required to have high machinability and low cost.
For example, Japanese Laid-Open Patent Publication No. 2009-24265 proposes an Al alloy including Mg in a range of 0.3 mass % to 6 mass %, Si in a range of 0.3 mass % to 10 mass %, Zn in a range of 0.05 mass % to 1 mass %, Sr in a range of 0.001 mass % to 0.3 mass %, and the remainder being Al and impurities.
In addition, International Publication Pamphlet No. WO2016/068293 proposes an Al alloy substrate for a magnetic disk, including Si in a range of 0.5 mass % to 24.0 mass % or less, Fe in a range of 0.01 mass % to 3.00 mass %, and the remainder being Al and unavoidable impurities.
Further, Japanese Laid-Open Patent Publication No. 6-145927 proposes a method of manufacturing a rolled plate for a magnetic disk, made of an Al—Mg alloy. This method performs continuous casting on a thin plate made of an Al—Mg alloy including 0.1 wt % Zr or less to a thickness in a range of 4 mm to 10 mm, and performs cold-roll at a reduction of 50% or higher without soaking the cast metal plate. Thereafter, annealing is performed at a temperature in a range of 300° C. to 400° C., to thereby manufacture a rolled plate having an average crystal grain diameter of 15 μm or less at a surface layer portion of the rolled plate. The Al—Mg alloy includes Mg in a range of 2.0 wt % to 6.0 wt %, at least one of Ti and B in a range of 0.01 wt % to 0.1 wt %, and at least one of Cr in a range of 0.03 wt % to 0.3 wt % and Mn in a range of 0.03 wt % to 0.3 wt %.